JUNE 2020 PCWorld 33The new specification will allow for
supported laptops to connect up to three 4K
(3840×2160) displays, each running at 90Hz,
and with 30 bpp (bits per pixel) 4:4:4 HDR
(no compression). The spec will also allow
you to connect a single 10K (10240x4320)
60Hz display without HDR, or a single 16K
(15360x8640) display with 30 bpp HDR,
though the latter capability requires
compression. Our earlier story (go.pcworld.
com/d2ln) has more detail.
The enabling technology is what VESA
calls DisplayPort Alternate Mode 2.0. While
using DisplayPort Alt Mode 2.0, the USB-C
connector can transmit up to 80Gbps of
DisplayPort video data utilizing all four high-
speed lanes in the cable, or up to 40Gbps
while using the other lanes for USB data
delivery to storage devices and other
peripherals. (The real-world throughput
is more like 77Gbps after encoding is
factored in.)
Put another way, common DisplayPort 1.4
devices, using DisplayPort cables, can
transmit 4 channels of data at 8.1Gbps
apiece. DisplayPort 2.0 via USB-C (Alt Mode)
ups that to four channels of 20Gbps apiece.
Finally, DSC 1.2a, the same advanced
compression protocol used in DisplayPort
1.4a, can be used to double that bandwidth
yet again.
While DisplayPort’s new Alt Mode spec
will run over the same USB-C connection as
your laptop may have already, changes under
the hood will mean that you’ll need to take
some care when buying and configuring
hardware. For one thing, USB-C devices
connected to your laptop communicate
among themselves, so any devices attached
to a USB-C hub have to share the available
bandwidth. Fortunately, there’s enough
available bandwidth that this won’t matter,
unless you’re also connecting to multiple,
high-resolution monitors.
Second, you’ll likely have to buy new
cables. Most cables supporting the new Alt
Mode, according to Anandtech (go.
pcworld.com/almd), will require some
active redrivers in the cable to enable the
highest resolutions. If the cable can’t
deliver enough bandwidth, your laptop will
negotiate to a lower-bandwidth signaling
level, probably without directly informing
you. That will mean that your shiny new 10K
display won’t output correctly.
It’s also not clear whether laptop
manufacturers will use the proper logo to
differentiate between the new DisplayPort Alt
Mode 2.0 devices and other ports. Both
DisplayPort 2.0 and USB4 are based on
Thunderbolt 3 signaling, which means
there’s enough commonality between them
to help enable the new standard to move
forward. Still, you’ll probably need to read
your future laptop’s manual carefully, as well
as any reviews that make note of the new
features, as this technology begins rolling
out next year.